Strategies to compensate for the effects of nonresponse on forest carbon baseline estimates from the national forest inventory of the United States

Abstract

Forest ecosystem carbon (C) stocks and stock change in the United States (US) have been documented using Intergovernmental Panel on Climate Change (IPCC) procedures and guidance with 1990 as a baseline reference for all United Nations Framework Convention on Climate Change reports. In the US, estimates of forest C stocks and stock change are obtained from data collected and maintained by the Forest Inventory and Analysis (FIA) program of the US Forest Service. Over the course of the IPCC monitoring period, the FIA program made a transition from state-by-state multiyear periodic inventories selected on a rotating basis – with reporting standards largely tailored to regional requirements – to nationally consistent, annual inventories (where a proportion of plots is measured in each state each year) designed for large-scale strategic requirements. Lack of measurements on all forest land during the periodic inventories, along with plot access difficulties and misidentification of forest plots as nonforest due to poor aerial imagery, have resulted in missing data (i.e., nonresponse) throughout the FIA database. Nonresponse, which in some US states is greater than 20%, may lead to differences in estimates of forest C stock change due to the procedural transition from periodic to annual inventories. As an initial step towards rectifying the differences in estimates, we examined several strategies to compensate for missing observations using the most recent annual inventory data from the Lake States region of the US. Results varied by state in the study but given the annual reporting cycle and requirements to compile national estimates of forest C, it was deemed that techniques, where non-observed samples are removed from estimation procedures, provided the optimal combination of statistical performance and efficiency. While the initial analysis focused on the Lake States region, several compensation strategies described may be useful in bridging the gap between national C flux estimates from periodic and annual forest inventories.